EMC implementation - Communication networks

Communication networks cover large distances and interconnect equipment installed in rooms that may have distribution systems with different system earthing arrangements. In addition, if the various sites are not equipotential, high transient currents and major differences in potential may occur between the various devices connected to the networks. As noted above, this is the case when insulation faults and lightning strikes occur. The dielectric withstand capacity (between live conductors and exposed conductive parts) of communication cards installed in PCs or PLCs generally does not exceed 500 V. At best, the withstand capacity can reach 1.5 kV. In meshed installations with the TN-S system and relatively small communication networks, this level of withstand capacity is acceptable. In all cases, however, protection against lightning strikes (common and differential modes) is recommended.
The type of communication cable employed is an important parameter. It must be suited to the type of transmission. To create a reliable communication link, the following parameters must be taken into account:

• Characteristic impedance
• Twisted pairs or other arrangement
• Resistance and capacitance per unit length
• Signal attenutation per unit length
• The type(s) of shielding used

In addition, it is important to use symmetrical (differential) transmission links because they offer higher performance in terms of EMC.
In environments with severe EM conditions, however, or for wide communication networks between installations that are not or are only slightly equipotential, in conjunction with IT, TT or TN-C systems, it is highly recommended to use optical fibre links.
For safety reasons, the optical fibre must not have metal parts (risk of electric shock if the fibre links two areas with different potentials).